Tumors are known to arise from normal cells through a series of stepwise transformations. Activation of signaling molecules and in particular growth factor related pathways could lead to malignant transformation of normal cells. Cancer mortality can be linked to the ability of tumors to undergo metastatic spread. The spread of tumors from the original site and the ability thereof to home in to specific tissues likely involves multiple steps as tumors are progressing from a non-invasive to an invasive state.
PTHrP was initially discovered as a mediator of malignancy associated hypercalcemia due to PTHrP's strong sequence homology at its amino or N-terminus with parathyroid hormone (PTH) at PTH's amino terminal end. The majority of patients with advanced cancer and hypercalcemia have been shown to have elevated circulating levels of PTHrP with or without associated osteolytic skeletal metastasis.
PTHrP is associated with the great majority of malignancies in the context of hypercalcemia including breast, colon, skin, renal and lung as well as hematological malignancies such as lymphomas, leukemias and multiple myelomas. Even more significant, in the absence of hypercalcemia and of elevation of circulating PTHrP levels, the expression of PTHrP in these tumor tissues has been shown to be elevated. Furthermore, several studies indicate that PTHrP may be a prognostic indicator in cancer patients and correlates with the metastatic process in several types of cancer including breast, prostate and colon cancer. Several studies suggest that PTHrP stimulates invasion in vitro and bone metastasis in vivo. The mechanism underlying PTHrP stimulation of bone metastasis is believed to be indirect by activating osteoclastic bone resorption and the release of local growth factors within the bone microenvironment that in turn stimulate growth of tumor cells within bone. The main target for treating bone metastasis in patients currently uses agents that reduce osteoclastic activity such as the class of agents known as bisphosphonates. PTHrP inhibition has therefore been identified as a potential target to inhibit osteoclastic activity within bone by reducing PTHrP production of tumor cells within bone. Monoclonal antibodies (“mAbs”) directed at the N-terminus of PTHrP have been used successfully in reducing osteolytic bone metastasis in nude mice transplanted with the human cell line MDAMB231. Humanized monoclonal antibodies directed at the N-terminal end of PTHrP have been generated and shown to be effective in nude mice models of hypercalcemia and bone metastasis. Clinical trials in patients with osteolytic bone metastasis with humanized monoclonal PTHrP antibodies directed to the N-terminus are currently underway.
In addition to its indirect effect on the bone metastatic process, several studies suggest that PTHrP may directly affect the growth and invasive abilities of tumor cells. Most of these studies were conducted in vitro and tend to indicate that PTHrP stimulates invasion and migration in different cell lines including breast, prostate and melanoma. In vivo data aside from studies on bone metastasis are very limited. One study indicate that PTHrP may be responsible for the growth of renal cancers and that growth and maybe metastasis is reduced by the administration of an antibody directed at the N-terminal end of PTHrP in nude mice transplanted with a human renal cancer cell line.
Because the PTH-like activity of PTHrP appears to lie within the N-terminal portion of the molecule, studies have used N-terminal fragments for in vitro and in vivo studies, particularly for studies of the PTH/PTHrP receptor Type-1 which can be activated by both PTH and PTHrP 1-34. This receptor has 7 transmembrane domains linked to G-Proteins and belongs to G-Protein coupled receptors (GPCRs). Ligand binding results in activation of both adenylate cyclase (cAMP pathway) and phospholipase C (PLC). Another PTH receptor (Type-2) has been cloned, is activated by a different ligand called TIP and is found mainly in the central nervous system whereas PTHrP Type-1 Receptor is ubiquitously expressed in most tissues. Furthermore, both the PTHrP Type-1 receptor and PTHrP are expressed simultaneously in the majority of breast carcinomas and this co-expression predicts poor survival.
The gene structure of human PTHrP is far more complex than PTH spanning over twenty (20) kilobases (kb) of genomic DNA and alternative mRNA splicing thereof gives rise to three isoforms of one-hundred and thirty-nine amino acids (139), one-hundred and forty-one amino acids (141) and one-hundred and seventy-three (173) amino acids. There is strong sequence homology between species but alternate splicing has not been reported in the lower species except for the canine gene. The mouse, rat, rabbit, bovine and chicken genes may only give rise to the isoform comprised of one-hundred and thirty-nine (139) amino acids. There is considerable divergence among species in the C-terminal end of PTHrP beyond amino acid 111. The long form, PTHrP1-173 may be unique to humans but its function is currently unknown although it has been suggested to play a role in cartilage growth. Antibodies directed to the N-terminus of PTHrP typically recognize all isoforms of PTHrP.
Despite the many years of research in this area, however, it remains to find the role of PTHrP and particularly its isoforms in diagnosis and treatment of disease, particularly cancer, tumor metastasis, osteolytic bone metastasis and hypercalcemia.